In the exhaust purification device of an engine which uses the SCR catalyst, urea water is supplied into an exhaust passage from the urea injector disposed upstream of the SCR catalyst in the exhaust passage. When the urea injector becomes a high temperature by being exposed to an exhaust gas of high temperature which flows through the exhaust passage, urea water in the urea injector boils. This boiling is a kind of small explosion generated inside the urea injector, and there may be a case where a valve or a metal part disposed inside the urea injector is damaged by such an explosion. Further, there may be also a case where moisture in urea water is dissipated by such boiling so that urea precipitates thus lowering sliding property of the valve.
As a countermeasure to prevent occurrence of such a damage, there has been known a technique in which, in a situation where a urea injector reaches a high temperature, an amount of urea water injected from the urea injector is increased so as to cool the urea injector by urea water itself. However, when an amount of urea water is increased, there may be a case where urea containing ammonium whose amount exceeds an amount of ammonium adsorbable by an SCR catalyst is supplied to an exhaust passage so that an excessively large amount of ammonium passes through the SCR catalyst. In this case, ammonium reacts with a slip catalyst disposed downstream of the SCR catalyst in the exhaust passage so that ammonium is reduced to NOx thus giving rise to a drawback that a discharge amount of NOx is increased. To overcome this drawback, JP2014-009661A discloses a technique in which, when a condition where a urea injector becomes a high temperature is satisfied, urea water is recovered into a urea tank from the urea injector.
However, when urea water is temporarily recovered into the urea tank from the urea injector, there is a concern that a state is brought about where no urea water exists in the urea injector and a urea water supply pipe. In this case, there is a concern that the supply of urea into the exhaust passage from the urea injector which takes place thereafter is delayed. When the supply of urea is not performed in a timely manner, a NOx purifying performance is lowered.